Inks having low rub-off properties are known in the prior art. Such inks typically contain waxes of various types and the resultant inks will exhibit improved mar-resistance and better slip and water repellency properties. Wax of a controlled fine particle size can be mixed or ground into the batch along with pigments or may be introduced during the final blending operations. Alternatively, the wax may be compounded into a "wax media" by dispersing or melting the wax into varnishes and/or solvents and adding these to the ink.
It is generally well-known that the non-rub qualities imparted by an individual wax are a function of both the particle size and the hardness as well as the melting temperature of any particular wax. However, the addition of wax to solve the rub-off problem introduces other problems. First, on a scale of 100 which represents an ink having no rub-off, when wax is added to ink the result is a reduction in rub-off to a level of only about 60. Second, with the heat and movement imparted by the friction of constant rubbing under pressure, particles of the ink film can ball up and mark unprinted areas. Additionally, introducing more wax to improve rub resistance properties only introduces more problems with respect to gloss and hardness characteristics. The addition of wax to ink almost invariably decreases the ink's level of gloss. Accordingly, a compromise must be achieved between the desired level of non-rub properties and gloss. Finally, wax only provides minimal rub-off reduction in news ink formulations.
Synthetic waxes such as polyethylene wax and polytetrafluoroethylene wax are the most popular waxes used in the ink industry. Such waxes are usually added in the form of "non-rub" or "slip" media which are fine dispersions of the wax in the solvents, oils and resins of the particular type of ink formulation in which it is to be incorporated. Waxes prepared from polytetrafluoroethylene powders are suitable for all types of printing inks, but are especially ideal for heatset inks, where the temperature of the drying apparatus does not cause the wax to soften or melt. Polytetrafluoroethylene-based waxes can also be stirred into finished inks to improve their rub and scuff resistance. Nevertheless, the problem is not completely eliminated by these strategies, particularly with regards to transit marking rub-off of magazine covers.
Another method used in the prior art to remedy the rub-off that may occur during transit is the use of insoluble resin, or oxidative chemistry based on oxidizable resin, drying oil and metallic driers.
Heatset inks that employ commercially available lithographic solvent soluble resin chemistry have not been able to eliminate the rub off problem, particularly when they are used to print on highly oil absorbent paper. In order to solve the problem, oxidative chemistry is utilized to achieve a degree of polymerization necessary to render an ink film that will not resoften twenty four (24) to forty eight (48) hours after heatset drying due to the hydrocarbon solvents being trapped within the printed substrate.
One problem associated with inks prepared by heatset oxidative chemistry is skin formation. Skin formation occurs, for example, when the ink is placed in tote bins and pumped to the printing press. A layer or ink film will form in the tote bins or in the pumping lines that feed the press.
Another solution to solving the ink rub-off problem is to use free radical polymerized ink resins suitable for use with ultraviolet or electron beam curable inks. These inks contain no volatile solvent and achieve a high molecular weight cross-link density when cured. However, the use of curable inks is expensive and thus not widely used.